ActiveRecord 4.1 Support (Fixing #91)

AR removed has_and_belongs_to_many and internally changed it into has_many through relationship. We need to be able to cope with this. Problem is in the ```#has_many_through_join``` method of ```QueryBuilder```.

Going to have more of a look at it later. First just adding this to gemfiles and travis. We still need to test against 4.0 because it still has the old has_and_belongs_to_many implementation.